Rauchgasseitige Korrosion von Nickelbasislegierungen für zukünftige Dampfkraftwerke

Lüttschwager, Frank

Dissertation / PhD Thesis/Book

Rauchgasseitige Korrosion von Nickelbasislegierungen für zukünftige Dampfkraftwerke

Lüttschwager, F. (Corresponding author)FZJ*

2011
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-89336-773-3

Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich : Energie & Umwelt / Energy & Environment 132, 145 S. (2011) = RWTH Aachen, Diss., 2011

Please use a persistent id in citations: http://hdl.handle.net/2128/4559

Abstract: Coal is still the most important energy source in Germany. In 2009 it produced 42.9 % of the overall German electrical power. Coal is available world-wide in large quantities and can be delivered economically. One of the possible ways to reduce CO$_{2}$ pollution is the increase of efficiency of coal fired power plants, which requires steam conditions of up to 700 °C – 730 °C and 350 bar. Because many German power units will reach the end of their technical lifespan in a few years or the following decade, one will have the possibility to build up modern types of power plants with increased efficiency of more than 50 %. Some international standards (European Pressure Equipment Directive or ASME Boiler and Pressure Vessel Code) require 100 000 hour creep rupture strength of 100 MPa at 750 °C. Therefore, nickel base alloys are in the focus of material qualification processes. Nickel base alloys are well investigated due to their hot corrosion behaviour. It is known that sodium sulphate may generate hot corrosion on those alloys at temperatures above its melting point of 884 °C. On nickel base alloys an eutectic mixture of nickel sulphate and sodium sulphate with a melting point of 671 °C can be generated, which leads to accelerated corrosion. This work examines, whether the high amount of sulphur and alkali metals will induce hot corrosion at the estimated working temperature on devices manufactured from nickel base alloy. Two synthetic coal ash deposits, according to the chemical composition of hard coal and lignite, and typical flue gases with and without sulphur dioxide were blended of pure agents. The reactions of the deposits with heater tubes´ materials and synthetic flue gases are examined in the temperature range from 650 °C to 800 °C and different time ranges up to 2000 hours. The corroded specimen are examined with SEM/EDX to identify relevant corrosion products and determine the corrosivity of deposited compounds. Deposits increase the corrosion rate of nickel base alloys in comparison to air oxidation, even when there is no gaseous sulphur oxide, like SO$_{2}$ or SO$_{3}$. This effect may be found by examining model systems, for example inert deposits like metal oxides, silicates or aluminosilicates and corrosive deposits like alkali sulphates. Kinetic investigations are carried out with a newly developed electrochemical pickling method, which allows to calculate the rates of hot corrosion underneath deposits. A material ranking is given for the alloys 263, 617 and 740, and it can be shown that under coal firing conditions lignite ash is much more corrosive than hard coal ash. Finally, thermodynamical calculations show possible reaction products formed during the combustion of various world market coals in power units. The coals’ potential of corrosion attacks is evaluated qualitatively by using experimental and thermodynamical data.

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